1. Technical Field
The invention relates to inductive antenna plasma reactors.
2. Background Art
Conventional inductively coupled plasma reactors typically have a large coiled inductive antenna to provide RF power for generating plasma. The large coil antenna, typically is located outside the reactor chamber and couples RF power through the chamber wall. Such a configuration has several drawbacks.
One disadvantage of this type reactor is that the reactor configuration limits location and efficiency of plasma generation. Due to the small skin depth, most of the power will be coupled to the chamber close to the chamber wall. Although the shape of the chamber might be changed to move the coil, and so the region of high power deposition, nearer to the workpiece, there are limits to how the chamber can be shaped in an attempt to bring the regions of high power deposition to the most advantageous location in relation to the workpiece. These limits derive from the fact that the shape of the chamber also has a significant impact on the characteristics of the plasma and the processing parameters associated therewith. Thus, a compromise must be struck between the shape of the chamber and the desired power deposition pattern therein. Typically, this precludes optimizing the power deposition within the chamber.
Another drawback with conventional inductive reactors is that process gas species dissociation can not easily be controlled. Typically, several plasma precursor gases are used to form the plasma. The composition of the generated ions or radicals from the mixture of the gases will depend on the dissociation and ionization energies of the constituent gases. If two precursor gases with substantially different breakdown voltages are used, primarily the precursor gas with the lower breakdown voltage will breakdown. For example, if both Cl.sub.2 and He are supplied to the chamber, the Cl.sub.2, which has a breakdown voltage of about a 9-10 eV, will dissociate and form ions, while He, which has a breakdown voltage of about 20 eV, essentially will not ionize. This is true even if inductive power is increased. As such, conventional reactors limit the types of precursor gases, and thus, the plasma species which may be used to process the workpiece.
Yet another problem with conventional inductive reactors is that conductive material may deposit on the walls of the reactor and degrade the efficiency and performance of the reactor. The conventional inductively coupled etch reactor has in the past been used to etch aluminum from the surface of a workpiece. This etching process produces byproducts comprising mostly aluminum chlorides (AlClx) and fragments of photoresist, which tend to deposit on the walls of the reactor chamber. The byproducts of an aluminum etch have no significant effect on etch rates because they are almost totally non-conductive. Such is not the case when electrically conductive etch byproducts are produced and deposited on the chamber surfaces. For example, etching of copper (Cu), platinum (Pt), tantalum (Ta), rhodium (Rh), and titanium (Ti), among others may create electrically conductive etch byproducts. Etching these metals presents a problem when using the conventional inductively coupled reactor.
Conductive deposits on the reactor walls can degrade reactor performance in several ways. Conductive deposits on the wall can reduce inductive power coupling to the plasma. The ceiling and upper portion of the side wall of the reactor chamber typically are made of a non-conductive material, such as quartz, to facilitate the coupling of power from the inductive coil antenna to the plasma. A coating formed by the conductive material on the walls and ceiling of the chamber has the effect of attenuating the inductive power coupled to the plasma.
As the interior surface of the chamber under the antenna is coated with a conductive material, eddy currents are produced in the material which attenuates the power coupled to the plasma. As the conductive coating builds in thickness over successive processes, the attenuation progressively increases and the power coupling into the plasma progressively decreases. It has been found that a 10 to 20 percent decrease in power coupled into the plasma occurs after etching 100 workpieces. Such a reduction in inductive power coupling into the chamber reduces the etch rate and can even cause problems igniting and maintaining a plasma.
The conductive coating also can cause unexpected changes in the characteristics of applied bias power. Typically, the lower portion of the reactor walls are made of an electrically conductive material, and are grounded to form an anode of a bias circuit used to control ion energy at the workpiece. The characteristics of the bias circuit, which controls ion energy at the workpiece, are particularly important during etching, as etching is ion driven.
The conductive coating formed on the insulated portion of the chamber walls can electrically connect to the grounded anode portion of the chamber. This effectively increases the anode area and results in an unexpected change in the bias power.
The reduction of inductively coupled power and the increase in capacitive bias power have detrimental effects on the etching process. The plasma ion density is lowered due to the decrease in inductively coupled power, and the plasma ion energy is increased due to the increase in capacitive bias power. As the power levels typically are set prior to the etching process to optimize plasma ion density and energy, any change could have an undesirable impact on etch quality. For instance, photoresist selectivity is lowered, etch stop depths are reduced, and ion current/energy distribution and the etch rate are adversely affected. Furthermore, it has been found that even after only two or three workpieces have been etched, unwanted changes in the etch profile can be observed.
Of course, the decrease in inductively coupled power could be compensated for by increasing the inductive power supplied to the inductive antenna. Similarly, the increase in capacitively bias power can be compensated for by decreasing the power supplied to the pedestal. Or, the chamber walls could be cleaned more often than would typically be necessary when etching materials producing non-conductive by-products.
These types of work-arounds, however, are generally impractical. A user of an etch reactor typically prefers to set the respective power levels in accordance with a so-called "recipe" supplied by the reactor's manufacturer. Having to deviate from the recipe to compensate for the conductive deposits would be unacceptable to most users. Furthermore, it is believed that the aforementioned detrimental effects will be unpredictable, and therefore, the required changes in the power settings could not be predetermined. Thus, unless the user employs some form of monitoring scheme, the required compensating changes in power inputs would be all but impossible for a user to implement. Realistically, the only viable solution would be to clean the chamber frequently, perhaps as often as after the completion of each etch operation. An increase in the frequency of cleaning, however, would be unacceptable to most users as it would lower throughput rates and increase costs significantly.
Another problem with conventional inductively coupled reactors is that the ratio of the surface area of the anode portion of the wall to the pedestal is too small. Since a large portion of the wall must be electrically non-conductive to facilitate inductive power coupling to the plasma, only a small portion of the wall is electrically conductive and may act as the anode for the capacitive bias supplied by an RF power source. It is desirable to have the surface area of the pedestal significantly less than that of the grounded portion so that the average voltage (often referred to as the DC bias voltage) at the surface of the workpiece is negative. This average negative voltage is employed to draw the positively charged ions from the plasma to the workpiece. If, however, the surface area of the pedestal is only slightly smaller than the surface area of the grounded portion, as is typically the case in a conventional inductively coupled plasma etch reactor, the average negative voltage at the surface of the workpiece is relatively small. This small average bias voltage results in a weak attracting force which provides a relatively low average ion energy. A higher negative bias voltage value than typically can be obtained using a conventional inductively coupled plasma etch reactor is necessary to optimize the plasma ion energy so as to ensure maximum etch rate while not creating significant damage to the workpiece. Ideally, the surface area of the grounded portion of the wall would be sufficiently large in comparison with that of the pedestal so as to produce the maximum possible negative average voltage at he surface of the workpiece, i.e. one half the peak to peak voltage.
Yet another drawback associated with the conventional inductively coupled reactor involves the cooling of the walls of the chamber. Most processes typically are only stable and efficient if the chamber temperature is maintained within a narrow range. Since the formation of the plasma generates heat which can raise the chamber temperature above the required narrow range, it is desirable to remove heat from the chamber in order to maintain an optimum temperature within the chamber. This typically is accomplished by flowing coolant fluid through cooling channels formed within the conductive portion of the chamber wall. As it is not easy to form cooling channels within the insulative portion of the chamber walls, air is directed over the exterior of these walls. A problem arises in that the electrically insulative materials, such as quartz or ceramic, typically used to form the chamber walls also exhibit a low thermal conductivity. Thus, the chamber walls are not ideal for transferring heat from the chamber. As a result, the chamber temperature tends to fluctuate more than is desired in the region adjacent the insulative chamber walls because the heat transfer from the chamber is sluggish. Often the temperature fluctuations exceed the aforementioned narrow range required for efficient etch processing.
These excessive temperature fluctuations can cause another problem. As discussed previously, deposits will tend to deposit on the chamber walls during the etch process. In attempting to control the chamber temperature by air cooling the insulative chamber walls, the chamber wall temperature and the layer of deposits formed on the interior surface thereof, tends to cycle. This cycling causes thermal stresses within the layer of the deposited material which result in pieces of the material flaking off the wall and falling into the chamber. The loose deposit material can contaminate the workpiece, or it can settle at the bottom of the chamber thereby requiring frequent chamber cleaning.